1. Field of Invention
The present invention relates to a variable capacitor (varactor) structure and its method of manufacture. More particularly, the present invention relates to a high-frequency variable capacitor structure and its method of manufacture.
2. Description of Related Art
In this fast-changing society, wireless communication techniques are progressing at an unbelievably rapid pace and hence play an important role in data communication networks. Wireless communication is mostly conducted at very high frequency. Amongst the components used in high-frequency communication, variable capacitor (varactor for short) occupies a center stage.
In general, to change the capacitance of a variable capacitor, supply voltage to the variable capacitor is varied. When the capacitance of the varactor is changed, transmission or reception frequency of a high-frequency circuit is increased or decreased accordingly. Ultimately, wireless communication equipment containing this piece of high-frequency circuit is able to receive or transmit communication signals more accurately.
In the fabrication of high-frequency circuits, the metal-oxide semiconductor (MOS) devices and bipolar complementary metal-oxide-semiconductor (BiCMOS) devices are formed before the variable capacitor.
FIG. 6 is a schematic cross-sectional view of a conventional variable capacitor. In the conventional method of fabricating a variable capacitor, a substrate 10 having N+ buried layer 12 on top, an N-well 14 above the buried layer 12, a deep collector region 20 also above the buried layer 12 and a field oxide layer 16 above the substrate 10 is provided. P-type ions are next implanted into the N-well 14 to form a P+-doped region 18 near the surface of the N-well 14. Thereafter, metal silicide layers 22 and 24 are formed over the P+-doped region 18 and the deep collector region 20 respectively. A dielectric layer 26 is formed over the substrate 10. Contacts 28 and 30 are formed in the dielectric layer 26 in contact with the metal silicide layers 22 and 24 respectively. Up to this stage, a variable capacitor structure comprising of the deep collector 20, the N+ buried layer 12 and the N-well 14/P+-doped region 18 is established.
Since the quality factor (Q) of the variable capacitor is inversely proportional to its resistance and capacitance and the capacitance value is the required capacitance of this variable capacitor, the quality factor of the variable capacitor can be improved only by lowering the resistance of the variable capacitor. However, in a conventional variable capacitor structure, the resistance of the variable capacitor is largely affected by the high resistance of the N-well 14. Since resistance at the N-well 14 is difficult to reduce, further improvement in the operating efficiency of a variable capacitor is hard to come by.